Method for the purification of corrosive gases

ABSTRACT

The present invention relates to a novel process for the preparation of high-purity chemicals with an extremely low particle count, such as ammonia gas, hydrogen fluoride and hydrogen chloride, which are also used as aqueous solutions in semiconductor technology the corrosive gas is enriched with an absorbent which is miscible with the gas and in which impurities present in the gas are soluble, and the gas is subsequently subjected to membrane filtration.

[0001] The present invention relates to a novel process for thepreparation of high-purity chemicals with an extremely low particlecount, such as ammonia gas, hydrogen fluoride and hydrogen chloride,which are also used as aqueous solutions in semiconductor technology.

[0002] For the production of highly integrated electronic circuits(microchips), extremely high-purity gases and aqueous chemicalsolutions, inter alia, are required. Important solutions are: ammoniasolution from ammonia gas, hydrofluoric acid solution from hydrogenfluoride and hydrochloric acid solution from hydrogen chloride.

[0003] Pure solutions are generally prepared by freeing technical-gradechemicals from particulate impurities by static filtration and producinga solution having a desired concentration by means of ultra-high-puritywater. A purification process for hydrochloric acid using this method isdescribed, for example, in U.S. Pat. No. 5,846,387.

[0004] Ultrafiltration is to date the only and most effective process bymeans of which extremely small particles can be removed from gases.However, according to data disclosed in the literature, ultrafiltrationonly removes particles of high-molecular-weight organic compounds, andconsequently only particles having a size of >10 daltons can be removedin this way. By contrast, metal ions, but also salt-containingparticles, have particle sizes of <1 to 5 daltons. These particle sizescannot be removed by ultrafiltration. It has hitherto only been possibleto remove particles having diameters of less than 10 daltons byhyperfiltration or by reverse osmosis. Reverse osmosis is unsuitableboth for the purification of gases and of highly aggressive chemicalsowing to the precautions necessary in the process. Experiments withvarious reverse osmosis plants have shown that if the pH deviatessignificantly from neutral, the membranes swell and become impermeable,i.e. become “blocked”. In addition, the actual particle removal byreverse osmosis plants required solutions of different concentrations atthe semi-permeable separating membranes.

[0005] The absorption columns usually employed are also unsuitable forpurification of this type. On the one hand, there is a risk of gasbreakthrough in the case of wall flow in the column. Another problemconsists in inhomogeneous layering of the packing elements in thecolumn, which are employed to increase the surface area of theinterfacial layers. The actual design of the columns also represents amajor problem since this is predominantly carried out intuitively. Thereis no arithmetical method for the design of corresponding columns whichallows a prediction of the purification effect in the ppt range,especially as the aggressive gases to be purified interact both with thepacking elements and also with the solvents employed and the material ofthe columns.

[0006] It is known that purified and pressure-liquefied gases canadditionally be passed through evaporators, and the gas can subsequentlybe purified by absorption of metallic impurities via one or morescrubbers. The scrubbing liquid employed here is a solution of the samechemical which is saturated with ultra-high-purity water.

[0007] EP 856715 A2 describes a process for the removal of metallicimpurities from corrosive gases by passing the latter into a receiverafter generation of the gas phase. This process achieves both areduction in the metal ion concentration and also in the particle count,since the transfer of the gas phase takes place automatically due to thepressure difference without mechanical pumping.

[0008] All known processes or process arrangements for the production ofhigh-purity, low-particle solutions have very similar problems.

[0009] If the purified gas is fed directly into a receiver containingultra-high-purity water, very high gas velocities arise at the inletnozzle which cause abrasion in and at the nozzle and thus have theconsequence of particulate and cationic re-contamination of thesolution.

[0010] Since the dissolution of the purified gases in water is anexothermic reaction, precautions for good heat dissipation have to betaken.

[0011] Inadequate heat dissipation in the receiver is associated withconsiderable temperature variations and consequently also considerablepressure variations. This in turn results in concentration deviations inthe solution. It is therefore generally attempted to homogenise thesolution by introducing the gas directly into the solution andsimultaneously pumping the solution through a bypass zone. However, thissucceeds only incompletely since simple pumping is a static operationwhich causes layering of the solution in the storage tank. It does notcause completely homogeneous mixing of the solution.

[0012] In order to be able to employ chemicals such as ammonia,hydrofluoric acid and hydrochloric acid in the electronics industry andsemiconductor production, grades with the lowest possible metal ioncontent must be available.

[0013] Corresponding impurities are often washed out of the gas byabsorption with the aid of packed columns. Packing elements are employedin this operation in order to increase the exchange area betweenscrubbing liquid and gas and to extend the residence time of the gas inthe column. There is a danger in packed columns of unpurified gasbreaking through (wall flow of the gas), and of aerosols beingtransferred into the pure circuit owing to a limited efficiency of thedemister installed at the top of the column. The degree of purificationin columns, in particular in the case of mass transfer in the ppt range,can usually only be set intuitively by means of additions. The idealstate of individual stream threads of wherever possible the same length,equal flow rates and constant mixing ratios between liquid and gas isnot achieved fully in any column. For optimisation, a plurality ofcolumns are consequently often connected in series in order to be ableto ensure constant purification in the high-purity range.

[0014] The known purification processes for the preparation ofhigh-purity chemicals do not enable the removal of organic impurities,such as, for example, oils which are always present inpressure-liquefied technical-grade ammonia gases. Typical oil contentsin technical-grade ammonia gas are between 10 and 50 ppm/w. Withoutremoval of the oils, a significant reduction in organic impurities(“TOC”, total organic carbon) in the purified solutions therefore doesnot occur.

[0015] The object of the present invention is thus to provide a meanswhich can be employed in a simple manner or an inexpensive process whichcan be carried out in a simple manner for the purification of corrosivegases and for the preparation of purified solutions of these gases whichcan be carried out in a simple plant and gives products of constant,high-purity, low-particle quality and which does not have thedisadvantages described above. A further object of the invention is toprovide a corresponding process which can be used for the purificationof pressure-liquefied gases, such as, for example, ammonia, hydrofluoricacid or hydrochloric acid.

[0016] The object is achieved by a continuous process for the removal ofparticulate, metallic, ionic inorganic, ionic organic, but also nonionicorganic impurities, and salts or oils present in traces, from corrosivegases, in which the gas is enriched in at least one process step with anabsorbent which is readily miscible with the gas and in which theimpurities present in the gas are readily soluble, and the gas issubsequently subjected to membrane filtration, in which the purified gasstream flows through the membrane, and the absorbent enriched withimpurities is taken off continuously.

[0017] This purification process according to the invention has thefollowing process steps per se:

[0018] a) a chemical in the liquid state is converted into the gaseousstate in an evaporator,

[0019] b) the resultant gas phase is freed from coarse particulateimpurities and impurities which are partially in the form of an aerosolby means of a prefilter, the latter impurities being fed to a collectingtank (7),

[0020] c) by setting the pressure to a value in the range between 2 and8 bar, preferably 3 and 8 bar, and simultaneously reducing thetemperature to a value in the range from 20 to 50° C., further organicimpurities present in the gas stream are deposited,

[0021] d) the gas stream pre-purified in this way is passed through atleast two ultrafine filters with decreasing pore size which areconnected in series, with ultrafine particles present in the gas phasebeing removed by means of a membrane,

[0022] e) organic impurities still present are separated off with theaid of an organics separator and fed into a collecting tank (7),

[0023] f) an absorbent which is readily miscible with the gas to bepurified and has a surface tension of >50 dynes is fed to the gas streamvia a saturation tank,

[0024] g) the gas stream is subjected to membrane filtration, in whichabsorbent is removed, and is subsequently fed in a mixer into aconstantly circulating solution whose concentration is monitored andregulated by means of acoustic velocity measurement.

[0025] The process is carried out in accordance with the invention byemploying a chemical selected from the group consisting of ammonia,hydrogen fluoride and hydrochloric acid in the liquid state.

[0026] The process is carried out by setting the pressure and thesaturation of the gas with absorbent with the aid of a pressure reducerand a heat exchanger.

[0027] In the organics separator, low-boiling and high-boiling organicimpurities are separated from the resultant multiphase mixture bysetting the pressure and temperature in such a way that the low-boilingimpurities are in the form of a gas or vapour mixture and thehigh-boiling impurities are in the form of a condensed oil or aerosol,which are subsequently separated off by means of suitable membranes.

[0028] In accordance with the invention, a further pressure reducer forsetting the pressure to a value in the range from 1 to 6 bar isinstalled downstream of the organics separator.

[0029] The present invention thus relates to a corresponding process inwhich, in order to remove ultrafine particulate, metallic, ionicinorganic, ionic organic, but also nonionic organic impurities, andsalts or oils present in traces, use is made of membranes comprising ahydrophobic support material which corresponds to the process stepcharacterised by g).

[0030] In a preferred embodiment of the process, the absorbent used isultra-high-purity water.

[0031] In a particular embodiment of the process, a gas scrubber in theform of a packed column with integrated demister, which serves as buffervolume in the event of a fault, can be installed downstream of themembrane filtration in process step g).

[0032] The gas stream can be fed in process step g) into a mixer havinga plurality of inlet tubes, in which the solution is circulated andexothermic heat formed on dissolution is dissipated in a loop or one ormore heat exchangers.

[0033] In order to carry out the process, the concentration of thedissolved purified gas is monitored and regulated by acoustic velocitymeasurement combined with precise determination of the solutiontemperature.

[0034] In particular, the object of the present invention is achieved bythe use of a membrane which has a liquid film for the removal of bothdissolved and particulate impurities from corrosive gases.

[0035] The purification process according to the invention is carriedout continuously in various process steps connected in series, with theprocess steps being selected in such a way that together they result inoptimum removal of not only particulate, metallic, ionic inorganic,ionic organic, but also of nonionic organic impurities, and salts oroils present in traces. The individual process steps are either stepswhich are known per se to the person skilled in the art or novelpurification steps which, in combination with one another, result inpurification of corrosive gases which has hitherto not been achieved.The process enables impurities to be removed to such an extent that theyare no longer detectable or the concentrations are <10 ppt.

[0036] The process according to the invention can be described insimplified terms as a process into which the following separationmethods are integrated:

[0037] 1. Removal of organic impurities in the crude gas using specialseparators.

[0038] 2. Seeding of the gas stream by means of a specific absorbent inorder to build up a suitable interfacial layer.

[0039] 3. Removal of volatile acids, anionic impurities, such as, forexample, chlorides and nitrates, trace impurities, such as metal ions orlow-volatility salts, or other impurities of the gases by means of aliquid film.

[0040] 4. Removal of contaminated liquids through a hydrophobic membraneafter the purification has been carried out.

[0041] 5. Adjustment of the plant in order to prepare a high-puritysub-ppt solution.

[0042] 6. Measurement of the concentration by means of a sonic converterin a special tubular probe, with coupled control of the other processparameters (pressure, temperature and flow rates).

[0043] The purification according to the invention is carried out per seby converting pressure-liquefied chemicals into the gaseous state in anevaporator (1). In the case of HF, this is carried out at a temperatureof >19° C. and at atmospheric pressure. A pre-filter (2) is used to freethis crude gas from coarse particulate impurities, which settle on thefilter surface. At the same time, some of the aerosols present in thegas are already separated off by the prefilter (2). The aerosolsseparated off in this way are collected in a collecting tank (7).

[0044] In gases or in the pressure-liquefied gases, high-boiling organicimpurities, preferably oils and water from the preparation process, arein bulk or aerosol form, whereas low-boiling impurities are in the formof a gas or vapour mixture. Organic impurities of this type are removedusing the different vapour-pressure values of individual substances inmulticomponent mixtures. To this end, a certain pre-pressure and acertain temperature are set in the gas phase with the aid of a pressurereducer (3) and a heat exchanger (4). It has been found that goodseparation of organic impurities from the gas phase is achieved if thepressure here is in the range between 3 and 8 bar and the temperature isset to a value in the range from 20 to 50° C.

[0045] A first separation of organic impurities is carried out afterparticulate impurities, which are in the form of pipe deposits or dirtparticles, have been separated off in a first stage. The particulateimpurities are separated off by passing the gas through at least twoultrafine filters (5) connected in series which have decreasing poresizes in the direction of the gas flow.

[0046] After the gas stream has passed through the ultrafine filters orfilter membranes, it enters an organics separator (6). For separation ofthe impurities present in the form of an aerosol, the gas stream ispassed through various types of membranes selected depending on thepreparation process for the individual gases. The aerosols are divertedinto the individual membranes in such a way that droplets form when theaerosol streams combine. Through selection of suitable membranematerials which have a suitable pore structure, “entrainment” of thedroplets is prevented. The retained droplets fall, due to gravity, inthe direction of the condensate discharge and are collected in acollecting tank (7).

[0047] It has been found that the content of high-boiling impurities,i.e. so-called residual oils, in the gas stream after passing throughthe organics separator is below the detection limit, i.e. <1 ppb. Forexample, oils can no longer be detected by the analytical methodscustomary today in ammonia gas which has been purified by the processaccording to the invention.

[0048] Gaseous oil vapours or organic impurities, in the form of boundvolatile acids, and anionic impurities, such as chlorides and nitrates,metal ions present in traces and low-volatility salts, but possibly alsoultrafine particulate impurities of various chemical types which arepresent in extremely small traces, can be removed in a subsequentpurification stage. These impurities are removed in a modified membranefiltration process (10). In order to carry out this modified membranefiltration, the pressure of the gas stream is reduced further,preferably to a value in the range from 1 to 6 bar, in an additionalpressure reducer (8).

[0049] Furthermore, an absorbent is added to the gas stream by beingpassed through a saturation tank (9). In combination with a definedpressure drop and suitable temperature regulation, a condensed phase(absorbent) is subsequently able to build up on the membrane surface(10) given a suitable choice of membrane. Besides the setting of thepressure and the temperature regulation of the gas stream, the flowdirection of the gas plays a particular role here in order to achieve apressure gradient from top to bottom on the membrane surface. This is ofimportance for the formation of a uniform liquid film, which in turn hasan essential effect on the removal of the impurities.

[0050] Under suitable conditions, a saturated liquid film forms ascondensed phase on the membrane surface, with the saturationconcentration becoming established in accordance with the gastemperature and the differential pressure between the phase interfaceand the membrane pressure.

[0051] This condensed phase on the membrane surface acts in a similarmanner to a semipermeable separating membrane in a reverse osmosisplant.

[0052] A migratory movement from top to bottom occurs in the liquid filmon the membrane surface. As a consequence of the continuous inflow ofunpurified gas to which absorbent has been added, the removal ofimpurities takes place continuously towards the condensate discharge dueto droplet formation in the liquid film. If the process is carried outcontinuously, the inflowing gas passes through the liquid layer on themembrane surface in the equilibrium state, with mass transfer with the,absorbent taking place at the interfacial layer. Moisture or absorbentis in turn removed from the saturated gas on flow through thehydrophobic membrane.

[0053] Besides ultrafine particulate impurities of various types,including metallic particles, both inorganic and organic ionicimpurities, but also salts or oils present in traces, can be separatedoff by this modified membrane filtration.

[0054] This process step can be carried out using commercially availablemembrane films. These may consist of different materials and areselected depending on the chemical to be purified. Matched to thecorrosive gas to be purified, the impurities present therein and theabsorbent employed, a very wide variety of membranes are available tothe person skilled in the art for selection. It is possible to usemembrane filters with either hydrophilic or with hydrophobic,microporous, thin separating layers. The choice of membrane filter isparticularly dependent on the absorbent employed, since effectiveremoval of the impurities still present in the gas can only be carriedout if the material of the membrane filter is impermeable to theabsorbent in which the impurities are soluble, but the gas to bepurified is readily able to pass through the membrane filter. In thiscase, the absorbent can be removed from the gas stream together with theimpurities before the membrane filter.

[0055] It has been found that hydrophilic absorbents are particularlysuitable for the removal of polar and ionic impurities. A hydrophilicabsorbent which is saturated with impurities and gas can be removedparticularly well from the gas stream if use is made of membrane filterswhich consist of hydrophobic material. Suitable membranes are those madefrom a material selected from the group consisting ofpolytetrafluoroethylene, perfluoroethylene-propylene and polyethylenewhich has a suitable pore width and structure. Particularly suitable arecorresponding materials having a pore size of less than 3.5 μm,preferably having a pore size in the range from 2.5 to 1 μm.Corresponding membrane filters are commercially available in variousdesigns, and it is consequently readily possible for the person skilledin the art to select a suitable membrane filter depending on therequirements. When selecting a suitable filter, the pore size and porewidth are just as important as the molecular and structural constructionof the membrane. In combination with the hydrophobic properties of thematerial, these parameters make a particular contribution to theseparating and purifying action.

[0056] Hydrophilic absorbents which can be employed are both hydrophilicorganic and inorganic solvents. The choice of absorbent is dependent onthe impurities present in the gas stream. If the impurities presentallow it, use is preferably made of ultra-high-purity water as aninexpensive and easily produced absorbent. However, it is also possiblefor any other liquid whose surface tension is >50 dynes/cm, which isvery readily miscible in any concentration with the gas to be purifiedand which has an adequate absorption capacity for the impurities presentto be used in combination with a hydrophobic membrane filter. Thus, forexample, it is also possible to use water-miscible, hydrophilic organicsolvents, such as low-molecular-weight alcohols. These may be, forexample, i- or n-propanol.

[0057] Liquids having a surface tension of less than 50 dynes/cm areunsuitable in combination with a hydrophobic membrane filter. They wetthe hydrophobic support material and make it permeable to liquids. Thedesired separating action of the membrane is lost.

[0058] If it is advisable to employ membrane filters having hydrophilicmaterial properties for the desired separating effect, it is readilypossible for the person skilled in the art to select a suitableabsorbent which does not pass through the membrane filter, but at thesame time has adequate solubility for the impurities present in the gasstream and is readily miscible with the gas.

[0059] Since absorbents which form a liquid film before the membrane areconstantly removed from the continuous process, and unsaturatedabsorbent is constantly fed freshly to the inflowing unpurified gasstream, constant mass transfer takes place at the phase interface of theliquid film, enabling the above-mentioned impurities to be removed fromthe gas phase very effectively.

[0060] After the gas has passed through the liquid film, it passesthrough the membrane with release of absorbent. After the-gas has passedthrough the membrane, organic impurities in the gas pre-purified in thisway are no longer detectable or have been removed down to the sub-pptrange and are present in concentrations of <10 ppt.

[0061] If the gas throughput through the membrane from top to bottom isconsidered, it is noted that the throughput drops down to zero as aconsequence of liquid accumulation and the associated increase in flowresistance. In this region of the membrane, a “calmed zone” forms, inwhich the absorbent is fed to the condensate discharge and is dischargedfrom the process.

[0062] In order for the process to be carried out in the optimum manner,it is necessary to match the process parameters temperature, gas flowrate, feed of absorbent, gas pressure and pressure difference betweenthe membrane surface and the phase interface to one another, where itshould be taken into account that these values are to be matched to theproperties of the chemical to be purified. Continuous maintenance ofthese parameters should also be ensured through suitable measurement andcontrol devices in order to be able to carry out continuous purificationof the chemicals in the sub-ppt range.

[0063] It has been found that good results can be achieved in anindustrial plant if the gas flow rate is set in the range from 100 to500 kg/h. The temperature and pressure values necessary for thepurification can be pre-determined very well using conventional computerprogrammes and matched to the membrane filtration according to theinvention. A suitable absorbent can also be determined from theliterature by the person skilled in the art since in general theprevious history of the gas to be purified allows conclusions to bedrawn on the impurities present.

[0064] After the membrane filtration, the gas is fed to a gas scrubber(12), which serves as buffer in the event of a fault. If the membranefiltration process is carried out in the standard way, the gas is notsubjected to further purification in the gas scrubber. In particular inthe case of membrane fracture, this stage serves to protect thedownstream plant parts against contamination with contaminated,unpurified gas.

[0065] It has been found that the process according to the inventionenables gas purification to a sub-ppt quality which cannot be achievedby purification carried out in the usual manner by means of gasscrubbers.

[0066] The downstream gas scrubber is designed as a single-stage columnwith packing elements and demister. The washing liquid used is asaturated solution prepared from ultra-high-purity water and purifiedgas. The washing liquid is stored in a separate tank (13) andcirculated.

[0067] Purification of the gas can be followed either by pressureliquefaction of the gas and transfer into pressure bottles or by thepreparation of saturated solutions of the gas.

[0068] The latter is carried out in a so-called mixing zone.

[0069] This may be designed as a loop reactor, but can also have adifferent design. The essential feature is that the purified gas isbrought into contact with the solvent at a controlled temperature andunder controlled pressure conditions with the largest possible surfacearea of the solvent. This is carried out by feeding the gas to aconstantly circulating solution in a special mixer (14) which is matchedto the respective solvency of the gas.

[0070] It has been found that good results are achieved if theconcentration change at the mixing point is about 1-3%. This is achievedby setting a corresponding ratio between the amount of solutioncirculating and the gas flow rate. The solvent used in this case isusually ultra-high-purity water.

[0071] For the preparation of saturated solutions, various designs ofmixers are available to the person skilled in the art for selection. Asuitable mixer usually has a plurality of inlet tubes which are matchedin design terms both in number, shape-and length to the particular gas.The gas stream is split over the inlet tubes and introduced into thelow-concentration solution circulating constantly at the same pressureand throughput and dissolved at various points in the mixer. Theexothermic dissolution process produces heat, which has to be dissipatedvia one or more heat exchangers. If the mixer is designed as a loopreactor, these heat exchangers can be integrated in a simple manner.

[0072] In the process carried out continuously, regular concentrationdetermination is necessary in order to be able to correct any processparameters and eliminate or avoid faults in the process.

[0073] It is essential for the purification process according to theinvention that the concentration regulation does not take place in theusual manner via the gas flow rate, but instead via the solvent,generally ultra-high-purity water. If the regulation of theconcentration setting takes place via the gas flow rate, changes occurin the process parameters, such as pressure and throughput and thusprocess changes occur at the membranes of the purification stage (10).Regulated concentration setting by measurement and regulation of thepressure and the volume flow rate cannot easily be carried out reliably.

[0074] A simple, fast and direct method for determination of theconcentration is based on acoustic velocity measurements in the solutionand on-line evaluation thereof.

[0075] The acoustic velocity is dependent on the density and theadiabatic compressibility of the solution. Maintenance of the requiredconcentration deviation of <0.5% in the solution requires both accuratedetermination of the solution temperature and an accurate tubular probe,which is necessary for back-coupling of the acoustic velocity.

[0076] This problem is solved in accordance with the invention bycarrying out the requisite-temperature measurement not in the tubularprobe itself, but instead in a calming zone. The calming zone isdesigned as a pipe with a plastic which is a ‘poor’ heat dissipator. Thefact that the saturation behaviour of plastics in solutions with highvapour pressures varies during changes in pressure and temperatureresults in differentiated measurement behaviour in the acousticback-coupling of the tubular probe. This behaviour of plastics is takeninto account by always carrying out the process in the loop underidentical process conditions.

[0077] In order to counter the inertia of the regulation system onstart-up of the plant, a buffer tank (15) is installed in the loopreactor. Since the solution present in the loop is always subject to thesame process conditions, the concentration regulation can adapt itselfvia the solution already set without the concentration variations thatare otherwise usual on start-up of the plant occurring in the system.The buffer tank results overall in better control behaviour and evensout small variations in the solvent feed.

[0078] The solution is discharged continuously into the storage tanks(17) via fixed amount regulation.

[0079] For the production of the individual components of the plant,materials should be selected which are inert to corrosive attack by thegas stream to be purified, but also meet the purity requirements in theindividual stages. High-quality stainless steels, plastics, such asPTFE, PVDF, PFA and PE, can be employed. In particular after the firstpurification stages in the pure region, the plant is made ofhigh-quality plastics.

[0080] For better understanding and in order to illustrate theinvention, a flow chart of a possible arrangement of a plant which fallswithin the scope of protection of the present invention is reproducedbelow by way of example. However, owing to the general validity of theinventive principle described, this is not suitable for reducing thescope of protection of the present application just to this example,since it is readily possible for the person skilled in the art to carryout variations in the construction of the plant, depending on the gas tobe purified, and to replace individual parts of the plant with deviceshaving the same action.

[0081] In flow chart 1, the construction of a plant according to theinvention is shown by way of example. This plant has the followingcomponents:

[0082] (1) Evaporator

[0083] (2) Pre-filter

[0084] (3) Pressure regulation

[0085] (4) Heat exchanger

[0086] (5) Ultrafine filter

[0087] (6) Organics separator

[0088] (7) Collecting tank

[0089] (8) Pressure tank

[0090] (9) Saturation tank

[0091] (10) Membrane filtration

[0092] (11) Condensate tank

[0093] (12) Gas scrubber

[0094] (13) Tank

[0095] (14) Mixer

[0096] (15) Buffer tank

[0097] (16) Heat exchanger

[0098] (17) Storage tank TABLE 1 Product quality of a 28% aqueousammonia solution prepared in accordance with the invention Crude productPurified NH₃ 100% [ppb] NH₄OH 28% [ppb] Al 3.8 <0.01 As <1 <0.01 Au <1<0.01 B 2.3 <0.01 Ba <1 <0.01 Be <1 <0.01 Bi <1 <0.01 Ca 7.4 <0.01 Cd <1<0.01 Co 1.0 <0.01 Cr 1.0 <0.01 Cu 1.5 <0.01 Fe 16.0 <0.01 Ga <1 <0.01Ge <1 <0.01 In <1 <0.01 K 4.6 <0.01 Li <1 <0.01 Mg 2.1 <0.01 Mn 1.0<0.01 Mo <1 <0.01 Na 5.7 <0.01 Ni 1.0 <0.01 Pb <1 <0.01 Pd <1 <0.01 Pt<1 <0.01 Sb <1 <0.01 Sn <1 <0.01 Sr <1 <0.01 Ti <1 <0.01 Tl <1 <0.01 V<1 <0.01 Zn 2.3 <0.01 Zr TOC >10.000 <501

[0099] TABLE 2 Composition of filter residues and impurities present inthe condensate which has been discharged at the membrane filterCollecting tank of Filter the separator [ppb] [ppb] Al 30 230 As Au <1<1 B 14 1250 Ba 4 <1 Be <1 <1 Bi <1 <1 Ca 560 85 Cd 4 270 Co 1 9 Cr 19140 Cu 180 7860 Fe 75 240 Ga <1 <1 Ge <1 <1 In <1 <1 K 170 1950 Li <1 28Mg 220 75 Mn 27 1000 Mo 39 440 Na 760 3270 Ni 240 900 Pb 1 1 Pd Pt <1 <1Sb Sn <1 <1 Sr 2 <1 Ti <1 <1 Tl <1 <1 V <1 <1 Zn 380 8780 Zr <1 <1 TOC7000 218000

1. Continuous process for the removal of particulate, metallic, ionicinorganic, ionic organic, but also nonionic organic impurities, andsalts or oils present in traces, from corrosive gases, characterised inthat the gas is enriched in at least one process step with an absorbentwhich is readily miscible with the gas and in which the impuritiespresent in the gas are readily soluble, and the gas is subsequentlysubjected to membrane filtration, in which the purified gas stream flowsthrough the membrane, and the absorbent enriched with impurities istaken off continuously.
 2. Continuous process according to claim 1 forthe removal of particulate, metallic, ionic inorganic, ionic organic,but also nonionic organic impurities, and salts or oils present intraces, from corrosive gases, characterised in that a) a chemical in theliquid state is converted into the gaseous state in an evaporator, b)the resultant gas phase is freed from coarse particulate impurities andimpurities which are partially in the form of an aerosol by means of apre-filter, the latter impurities being fed to a collecting tank (7), c)by setting the pressure to a value in the range between 2 and 8 bar,preferably 3 and 8 bar, and simultaneously reducing the temperature to avalue in the range from 20 to 50° C., further organic impurities presentin the gas stream are deposited, d) the gas stream pre-purified in thisway is passed through at least two ultrafine filters with decreasingpore size which are connected in series, with ultrafine particlespresent in the gas phase being removed by means of a membrane, e)organic impurities still present are separated off with the aid of anorganics separator and fed into a collecting tank (7), f) an absorbentwhich is readily miscible with the gas to be purified and has a surfacetension of >50 dynes is fed to the gas stream via a saturation tank, g)the gas stream is subjected to membrane filtration, in which absorbentis removed, and is subsequently fed in a mixer into a constantlycirculating solution whose concentration is monitored and regulated bymeans of acoustic velocity measurement.
 3. Process according to claims 1to 2, characterised in that the chemical in the liquid state is achemical selected from the group consisting of ammonia, hydrogenfluoride and hydrochloric acid.
 4. Process according to claim 2,characterised in that the pressure and the saturation of the gas withabsorbent are set with the aid of a pressure reducer and a heatexchanger.
 5. Process according to claim 2, characterised in that in theorganics separator, low-boiling and high-boiling organic impurities areseparated the multiphase mixture-by setting the pressure and temperaturein such a way that the low-boiling impurities are in the form of a gasor vapour mixture and the high-boiling impurities are in the form of acondensed oil or aerosol, which are subsequently separated off by meansof suitable membranes.
 6. Process according to claim 2, characterised inthat a further pressure reducer for setting the pressure to a value inthe range from 1 to 6 bar is installed downstream of the organicsseparator.
 7. Process according to claims 1 and 2, characterised in thatmembranes comprising a hydrophobic support material are used in processstep g) for removing ultrafine particulate, metallic, ionic inorganic,ionic organic, but also nonionic organic impurities, and salts or oilspresent in traces.
 8. Process according to claims 1 and 2, characterisedin that the absorbent used is ultra-high-purity water.
 9. Processaccording to claim 2, characterised in that a gas scrubber in the formof a packed column with integrated demister, which serves as buffervolume in the event of a fault, is installed downstream of the membranefiltration in process step g).
 10. Process according to claim 2,characterised in that the gas stream in process step g) is fed into amixer having a plurality of inlet tubes, in which the solution iscirculated and exothermic heat formed on dissolution is dissipated in aloop or one or more heat exchangers.
 11. Process according to claim 2,characterised in that the concentration of the dissolved purified gas ismonitored and regulated by acoustic velocity measurement combined withprecise determination of the solution temperature.
 12. Use of a membranewhich has a liquid film for the removal of both dissolved andparticulate impurities from corrosive gases.